The communication spectrum is becoming a more and more precious resource as a result of rapid development of global communication services. Aiming to use spectrum resources more effectively, modulation systems of high spectral efficiency, such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), have been adopted in communication systems to modulate phase and amplitude of carriers, which results in non-constant envelope modulated signals of large peak-to-average ratio (PAR). Even multi-carrier technology is being used for such non-constant envelope modulated signals. However, large PAR cannot be avoided during multicarrier synthesis, which results in poor linearity performance. Better linearity performance of transmitters leads to less interference between different equipment within the communication network and between carriers of different frequency within a single equipment. Thus, better linearity performance results in higher utilization of communication spectrum resources. As a result, there are higher demands for the linear adjustment of power amplifiers which are the core component in determining linearity performance of signals, in transmitters.
As the development of digital technologies and RF technologies grows, the solution to the linear adjustment issue of power amplifiers is to adopt digital pre-distortion technology. FIG. 1 shows a popular digital pre-distortion power amplifier system, that includes an I/Q baseband signal input port, a baseband data processor, a baseband signal clipping equipment, a baseband pre-distortion processor, a pre-distortion parameter adapter, a baseband data processor, a digital-to-analog (D/A) converter, an analog-to-digital (A/D) converter, up-converter equipment, local oscillator equipment, down-converter equipment, a power amplifier, a coupler, and an RF output port.
As shown in FIG. 1, the received I/Q baseband signals undergoes pre-distortion processing. In this regard, I/Q baseband signals are received at the I/Q baseband signal input port and processed by the baseband data processor and the baseband signal clipper equipment, which results in digital baseband signals with a lower PAR.
The digital baseband signals with a lower PAR go through the baseband pre-distortion processor, D/A converter, and up-converter equipment to become post-pre-distortion RF signals. The post-pre-distortion RF signals are then amplified through the high-power amplifier and output through the RF output port. To monitor the system linearity in real time, some RF signals are picked to go through the coupler at the RF output port. From the coupler, the RF signals go through the down-converter and A/D converter to become digital feedback baseband signal. After going through the baseband data processor and pre-distortion parameter adaptor control equipment, the signal implements the linearity adjustment on the lower-PAR digital baseband signal received by the baseband pre-distortion processor, which results in optimal linearity adjustment of the pre-distortion power amplifier system.
The digital pre-distortion system as shown in FIG. 1 can realize the linear requirements by passing the received I/Q baseband signals through the pre-distortion power amplifier system. However, the I/Q baseband signal input port used in the digital pre-distortion power amplifier system shown in FIG. 1 can only be used in certain equipment, such as base stations and remote radio units (RRU). It is impossible to use an RF port as the input port in the pre-distortion power amplifier system. This is problematic since a large proportion of the communication equipment used to provide current communication network coverage use an RF port as the signal input port. Based on the solution shown in FIG. 1, FIG. 2 illustrates a digital pre-distortion power amplifier system with an RF input port that has been developed.
The digital pre-distortion power amplifier system of FIG. 2 includes an RF signal input port, a first down-converter, a first local oscillator, a first A/D converter, a digital down-converter and filter processor, baseband signal clipping equipment, a baseband pre-distortion processor, a pre-distortion parameter adaptation controller, a baseband data processor, a D/A converter, a second A/D converter, an up-converter, a second local oscillator, a second down-converter, a power amplifier, a coupler, and an RF output port.
As shown in FIG. 2, the digital pre-distortion power amplifier system that includes an RF input port performs the same basic processing theories on received signals as the digital pre-distortion power amplifier system that is based on the I/Q baseband signal input port. The digital pre-distortion power amplifier of FIGS. 1 and 2 only differ in terms of the type of signal input port. Because of the different signal input ports, different kinds of signals are processed in the pre-distortion operation (e.g., a digital baseband signal in the case of an I/Q baseband signal input port versus an RF signal in the case of an RF input port). In comparing the two systems, the signal performance, such as signal frequency and signal quality, in the system using the digital baseband signal is superior to the system using the RF signal.
Moreover, in the pre-distortion power amplifier system shown in FIG. 2, the RF signals are processed jointly by the first down-converter and first local oscillator after being received through the RF input port, and then by the first A/D converter. If the pre-distortion power amplifier system shown in FIG. 2 is used for the pre-distortion treatment of the received RF signals, the signal linearity will be rather poor when the pre-distortion-treated signals are output through the power amplifier since the RF signals are received through the RF input port and have lower signal quality than the digital baseband signal. That may cause signals of bad quality in the coverage area and affect the normal operation of a communication network.